Evidence for a new kidney-produced hormone, 1,25-dihydroxycholecalciferol, the proposed biologically active form of vitamin D

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Abstract
It has been conclusively shown that vitamin D3(cholecalciferol) must first be metabolized prior to its mediating intestinal calcium transport. The first transformation is conversion of vitamin D3by the liver to 25-hydroxyvitamin D3. This compound is then subsequently metabolized by the kidney to 1,25-dihydroxyvitamin D3. 1,25-Dihydroxyvitamin D3is over four times as effective as vitamin D3and more than twice as effective as 25-OH-vitamin D3in stimulating intestinal calcium transport. Additionally, 1,25-dihydroxyvitamin D3is highly active in stimulating bone calcium resorption. As such, 1,25-dihydroxyvitamin D3likely represents the biologically active form of the vitamin in the intestine and bone. The secretion of this steroid by the kidney and its selective accumulation by the target organs supports the concept that this compound should be regarded as a hormonal regulator of calcium metabolism.